The present invention relates to a variable speed pumping-up system, and more particularly to a variable speed pumping-up system provided with a pump or a pump turbine which exhibits a stall characteristic with a reverse flow called a hump characteristic in an operating region of high total dynamic head.
The present invention relates to a variable speed pumping-up system, and more particularly to a method of controlling a variable speed pumping-up system sharing an upstream side pipe line or a downstream side pipe line with another hydraulic machine and having a pump or a pump turbine which exhibits a hump characteristic (a reverse flow characteristic) in a pump operation region.
It is considered that, in a pump turbine of a variable speed pumping-up system, the occurrence of the hump characteristic (wherein an operation of the pump turbine becomes dH/dQ&gt;0, wherein H is a total dynamic head including head loss in the upstream and downstream pipelines; and Q is a flow rate), which is shown by a region indicated by z in FIG. 7, in a higher total dynamic head operating region of the pump cannot generally be avoided.
The operation of the pump turbine in the hump characteristic region is unstable and causes vibrations and noises. Moreover, since a flow rate Q of water can never be stabilized in the range of z on the H versus Q curve of FIG. 7, an operating point of the pump turbine which has been approaching the point (Q.sub.X, H.sub.X) from the larger Q side changes suddenly to the point (Q.sub.Y, H.sub.X) as soon as the total dynamic head H reaches H.sub.X. Thus abnormal water hammering phenomena occur in the upstream and downstream pipe lines.
The degree of depression of the hump characteristic differs depending upon the degree of opening of a guide vane as shown in FIG. 8. Each degree of opening of the guide vane in FIG. 8 is pointed out as Y.sub.4 &lt;Y.sub.3 &lt;Y.sub.2 &lt;Y.sub.1. Even when a clear depression portion does not appear in the hump characteristic region, a similar problem occurs, more or less, since the flow of water in a runner of the pump turbine becomes unstable.
It is said that the cause of such a problem resides in that, when the total dynamic head of the pump turbine is too high under a given guide vane opening, the flow of water is remarkably reduced and distorted allowing partial reverse flows as shown in FIGS. 5 and 6. In this condition, the flow of water separates from the surface of the runner, and is put in a so-called stalling state.
Japanese Patent Laid-Open No. 186069/1987 includes a description about the effect that, when a load on a variable speed pumping-up apparatus is to be increased, the steps of initially increasing an output level of an electric drive unit so as to set a rotating speed of a pump turbine higher, and thereafter increasing a degree of opening of a guide vane should be taken for the purpose of preventing an operation point of the pump turbine from falling transitionally into the above mentioned hump characteristic region.
This publication also discloses a proposal that, when the load on the variable speed pumping-up apparatus is to be reduced, the closing of the guide vane and the decreasing of the rotating speed of the pump turbine are performed simultaneously so that the closing operation of the guide vane terminates earlier than the rotating speed decreasing operation of the pump turbine.
A case where the pump turbine shares its upstream side pipe line or downstream side pipe line with some other hydraulic machines, namely a case where a plurality of hydraulic machines are provided so that the hydraulic machines are connected to the pipe lines branching from a single pipe line, will now be considered.
When the operation condition of any of the additional hydraulic machines including the flow rate of water therein is changed, water hammering phenomena occur in this hydraulic machine and is transmitted to the pump turbine under consideration as well via the common pipe line, so that the pump turbine is necessarily influenced by the water hammering phenomena.
Especially, when the total dynamic head in an independent variable speed pumping-up system is swung higher, it is possible that the operation point of the pump turbine therein falls into the hump characteristic region even if the variable speed pumping-up system itself is set in a perfectly controllable region.
Japanese Patent Laid-Open No. 149583/1986 discloses a pump turbine starting method for a variable speed pumping-up system, in which, when a pump turbine is started, namely, when the pump turbine operation is shifted from a priming pressure established state with guide vanes fully closed to a regular variable speed pumping-up operation with a desired load, the rotating speed of the pump turbine is increased in proportion to the opening of the guide vanes, whereby a proper rotating speed of the pump turbine and a proper degree of opening of the guide vane with respect to the desired load are finally attained.
However, this known patent publication does not refer at all to a control method for a variable speed pumping-up system to be used after a regular variable speed pumping-up operation has been commenced.
As may be understood from the above stated description, the conventional controlling method for the variable speed pumping-up system for preventing the operation point of the pump turbine from falling into the hump characteristic region discloses only a part of the control needed for an independent variable speed pumping-up system.
Further, Japanese Patent Laid-Open No. 175271/1986 discloses a controlling method for a variable speed pumping-up system, in which, when the static head, i.e., difference between water level; of an upper reservoir and water level of a lower reservoir has a predetermined value, the rotating speed of the pump turbine is raised and corrected in accordance with an overshoot thereof.
However, nothing is referred to with respect to the operation point of the pump turbine falling into the hump characteristic (the reverse flow characteristic) region by a temporary and much faster increase of the total dynamic head of this pump turbine due to the water hammering phenomena in the shared pipe line to which the present invention is directed.
As may be understood from the above stated description, the conventional controlling method for the variable speed pumping-up system for preventing the operation point of the pump turbine from falling into the hump characteristic region discloses only a part of the control that needs to be applied to an independent variable speed pumping-up system.
Namely, a countermeasure of the operation point of the pump turbine falling into the hump characteristic region due to the water hammering phenomena in the shared pipe line has not yet been proposed at all.